How do you cache an image in Javascript

Question

My friends and I are working on a website where we would like to cache certain images in order to display them faster in the future   I have two main questions    How do you cache an image  How do you use an image once it has been cached   and just to verify  if an image is cached on page A  it is possible to call it from the cache to use it on page B  right     Also  is it possible to set when the cached version of the image will expire   It would be much appreciated if an example and or a link to a page which describes this further was included   We re fine using either raw Javascript or the jQuery version

User · Answer

Adding for completeness of the answers  preloading with HTML   lt link rel  preload  href  bg-image-wide png  as  image  gt    Other preloading features exist  but none are quite as fit for purpose as  lt link rel  preload  gt      lt link rel  prefetch  gt  has been supported in browsers for a long time  but it is intended for prefetching resources that will be used in the next navigation page load  e g  when you go to the next page   This is fine  but isn t useful for the current page  In addition  browsers will give prefetch resources a lower priority than preload ones     the current page is more important than the next  See Link prefetching FAQ for more details    lt link rel  prerender  gt  renders a specified webpage in the background  speeding up its load if the user navigates to it  Because of the potential to waste users bandwidth  Chrome treats prerender as a NoState prefetch instead    lt link rel  subresource  gt   was supported in Chrome a while ago  and was intended to tackle the same issue as preload  but it had a problem  there was no way to work out a priority for the items  as didn t exist back then   so they all got fetched with fairly low priority    There are a number of script-based resource loaders out there  but they don t have any power over the browser s fetch prioritization queue  and are subject to much the same performance problems   Source  https   developer mozilla org en-US docs Web HTML Preloading content

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Once an image has been loaded in any way into the browser  it will be in the browser cache and will load much faster the next time it is used whether that use is in the current page or in any other page as long as the image is used before it expires from the browser cache   So  to precache images  all you have to do is load them into the browser   If you want to precache a bunch of images  it s probably best to do it with javascript as it generally won t hold up the page load when done from javascript   You can do that like this   function preloadImages array        if   preloadImages list            preloadImages list                 var list   preloadImages list      for  var i   0  i  lt  array length  i              var img   new Image            img onload   function                 var index   list indexOf this               if  index     -1                       remove image from the array once it s loaded                    for memory consumption reasons                 list splice index  1                                   list push img           img src   array i            preloadImages   url1 jpg    url2 jpg    url3 jpg       This function can be called as many times as you want and each time  it will just add more images to the precache   Once images have been preloaded like this via javascript  the browser will have them in its cache and you can just refer to the normal URLs in other places  in your web pages  and the browser will fetch that URL from its cache rather than over the network   Eventually over time  the browser cache may fill up and toss the oldest things that haven t been used in awhile   So eventually  the images will get flushed out of the cache  but they should stay there for awhile  depending upon how large the cache is and how much other browsing is done    Everytime the images are actually preloaded again or used in a web page  it refreshes their position in the browser cache automatically so they are less likely to get flushed out of the cache   The browser cache is cross-page so it works for any page loaded into the browser   So you can precache in one place in your site and the browser cache will then work for all the other pages on your site     When precaching as above  the images are loaded asynchronously so they will not block the loading or display of your page   But  if your page has lots of images of its own  these precache images can compete for bandwidth or connections with the images that are displayed in your page   Normally  this isn t a noticeable issue  but on a slow connection  this precaching could slow down the loading of the main page   If it was OK for preload images to be loaded last  then you could use a version of the function that would wait to start the preloading until after all other page resources were already loaded   function preloadImages array  waitForOtherResources  timeout        var loaded   false  list   preloadImages list  imgs   array slice 0   t   timeout    15 1000  timer      if   preloadImages list            preloadImages list                 if   waitForOtherResources    document readyState      complete             loadNow          else           window addEventListener  load   function                 clearTimeout timer               loadNow                           in case window addEventListener doesn t get called  sometimes some resource gets stuck             then preload the images anyway after some timeout time         timer   setTimeout loadNow  t              function loadNow             if   loaded                loaded   true              for  var i   0  i  lt  imgs length  i                      var img   new Image                    img onload   img onerror   img onabort   function                         var index   list indexOf this                       if  index     -1                               remove image from the array once it s loaded                            for memory consumption reasons                         list splice index  1                                                           list push img                   img src   imgs i                                    preloadImages   url1 jpg    url2 jpg    url3 jpg    true   preloadImages   url99 jpg    url98 jpg    true

User · Answer

Even though your question says  using javascript   you can use the prefetch attribute of a link tag to preload any asset  As of this writing  Aug 10  2016  it isn t supported in Safari  but is pretty much everywhere else    lt link rel  prefetch  href   url   gt   More info on support here  http   caniuse com  search prefetch  Note that IE 9 10 aren t listed in the caniuse matrix because Microsoft has discontinued support for them   So if you were really stuck on using javascript  you could use jquery to dynamically add these elements to your page as well  -

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Nowdays  there is a new technique suggested by google to cache and improve your image rendering process    Include the JavaScript Lazysizes file  lazysizes js Add the file to the html file you want to use in   lt script src  lazysizes min js  async gt  lt  script gt  Add the lazyload class to your image   lt img data-src  images flower3 png  class  lazyload  alt    gt

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I use a similar technique to lazyload images  but can t help but notice that Javascript doesn t access the browser cache on first loading   My example   I have a rotating banner on my homepage with 4 images the slider wait 2 seconds  than the javascript loads the next image  waits 2 seconds  etc   These images have unique urls that change whenever I modify them  so they get caching headers that will cache in the browser for a year   max-age  31536000  public   Now when I open Chrome Devtools and make sure de  Disable cache  option is not active and load the page for the first time  after clearing the cache  all images get fetch and have a 200 status  After a full cycle of all images in the banner the network requests stop and the cached images are used    Now when I do a regular refresh or go to a subpage and click back  the images that are in the cache seems to be ignored  I would expect to see a grey message  from disk cache  in the Network tab of Chrome devtools  In instead I see the requests pass by every two seconds with a Green status circle instead of gray  I see data being transferred  so it I get the impression the cache is not accessed at all from javascript  It simply fetches the image each time the page gets loaded   So each request to the homepage triggers 4 requests regardless of the caching policy of the image   Considering the above together and the new http2 standard most webservers and browsers now support  I think it s better to stop using lazyloading since http2 will load all images nearly simultaneously   If this is a bug in Chrome Devtools it really surprises my nobody noticed this yet      If this is true  using lazyloading only increases bandwith usage   Please correct me if I m wrong

User · Answer

I have a similar answer for asynchronous preloading images via JS  Loading them dynamically is the same as loading them normally  they will cache   as for caching  you can t control the browser but you can set it via server  if you need to load a really fresh resource on demand  you can use the cache buster technique to force load a fresh resource

User · Answer

There are a few things you can look at   Pre-loading your images Setting a cache time in an  htaccess file File size of images and base64 encoding them     Preloading  http   perishablepress com 3-ways-preload-images-css-javascript-ajax   Caching  http   www askapache com htaccess speed-up-sites-with-htaccess-caching html  There are a couple different thoughts for base64 encoding  some say that the http requests bog down bandwidth  while others say that the  perceived  loading is better   I ll leave this up in the air

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Yes  the browser caches images for you  automatically   You can  however  set an image cache to expire  Check out this Stack Overflow questions and answer   Cache Expiration On Static Images

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I always prefer to use the example mentioned in Konva JS  Image Events to load images    You need to have a list of image URLs as object or array  for example   var sources                 lion    assets lion png               monkey    assets monkey png             Define the Function definition  where it receives list of image URLs and a callback function in its arguments list  so when it finishes loading image you can start excution on your web page     x000D   x000D      function loadImages sources  callback    x000D                  var images       x000D                  var loadedImages   0  x000D                  var numImages   0  x000D                  for  var src in sources    x000D                      numImages    x000D                    x000D                  for  var src in sources    x000D                      images src    new Image    x000D                      images src  onload   function      x000D                          if    loadedImages  gt   numImages    x000D                              callback images   x000D                            x000D                         x000D                      images src  src   sources src   x000D                    x000D                x000D   x000D   x000D     Lastly  you need to call the function  You can call it for example from jQuery s  Document Ready     document  ready function            loadImages sources  buildStage

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as  Pointy said you don t cache images with javascript  the browser does that  so this may be what you are asking for and may not be    but you can preload images using javascript  By putting all of the images you want to preload into an array and putting all of the images in that array into hidden img elements  you effectively preload  or cache  the images   var images       path to image1 png     path to image2 png        images  each function     var image       lt img   gt    attr  src   this

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